Immuno-gold localization of leghaemoglobin in cytoplasm in nitrogen-fixing root nodules of pea

A unique feature of nitrogen-fixing nodules formed by association of species of Rhizobium and Frankia with various legumes and non-legumes is that they contain leghaemoblogin (Lb), an oxygen-carrying myoglobin-like protein1–6. Elucidation of the function of Lb, which is thought to facilitate the flow of oxygen to the nitrogen-fixing bacteroid forms of Rhizobium within infected nodule cells7,8, has been hindered by uncertainty regarding its precise intracellular location9. Several workers have reported that Lb is located exclusively in the plant cytoplasm in nodules of various legumes10–12, while others13–15 have concluded that some of the Lb is located in the peribacteroid spaces in some legumes. Using immuno-gold staining16 of Lb on thin sections of pea nodules, we demonstrate here that Lb can be detected in the plant cytoplasm and the nucleus of infected cells but not in the peribacteroid spaces

The fhu genes of Rhizobium leguminosarum, specifying siderophore uptake proteins: fhuDCB are adjacent to a pseudogene version of fhuA

A mutant of Rhizobium leguminosarum was isolated which fails to take up the siderophore vicibactin. The mutation is in a homologue of fhuB, which in Escherichia coli specifies an inner-membrane protein of the ferric hydroxamate uptake system. In Rhizobium, fhuB is in an operon fhuDCB, which specifies the cytoplasmic membrane and periplasmic proteins involved in siderophore uptake, fhuDCB mutants make vicibactin when grown in Fe concentrations that inhibit its production in the wild-type. Nodules on peas induced by fhuDCB mutants were apparently normal in N2 fixation. Transcription of an fhuDCB-lacZ fusion was Fe-regulated, being approximately 10-fold higher in Fe-depleted cells. Downstream of fhuB, in the opposite orientation, is a version of fhuA whose homologues in other bacteria specify hydroxamate outer-membrane receptors. This fhuA gene appears to be a pseudogene with stop codons and undetectable expression

RirA, an iron-responsive regulator in the symbiotic bacterium Rhizobium leguminosarum.

Mutations in a Rhizobium leguminosarum gene, rirA (rhizobial iron regulator), caused high-level, constitutive expression of at least eight operons whose transcription is normally Fe-responsive and whose products are involved in the synthesis or uptake of siderophores, or in the uptake of haem or of other iron sources. Close homologues of RirA exist in other rhizobia and in the pathogen Brucella; many other bacteria have deduced proteins with more limited sequence similarity. None of these homologues had been implicated in Fe-mediated gene regulation. Transcription of rirA itself is about twofold higher in cells grown in Fe-replete than in Fe-deficient growth media. Mutations in rirA reduced growth rates in Fe-replete and -depleted medium, but did not appear to affect symbiotic N fixation